1. Field of the Invention
The present invention relates to an optical information memory medium and, more particularly, to an optical information memory medium using an organic pigment as a recording layer.
2. Description of the Prior Art
An example of the optical information memory medium is an optical disk. An optical disk consists of a disk-shaped substrate and a thin-film recording layer formed thereon. The recording layer stores information by forming an optically detectable small (about 1 .mu.m) pit. A plurality of pits are formed in the form of a helical or concentric track to record information at high density.
In order to write information in an optical disk, a laser beam is radiated so as to be focused on the surface of the recording layer having sensitivity with respect to the laser. Radiated laser energy is absorbed in the recording layer, and an optically detectable pit is formed. For example, in a heat mode recording method, a radiated portion of the recording layer absorbs the laser energy and hence is locally heated. This portion is melted and evaporated or agglomerates, i.e., physically changes. As a result, a difference is produced in an optically detectable property such as and a non-radiated portion. Therefore, information is written in a state to allow subsequent reading. Note that in order to form a plurality of pits in the form of a helical or concentric track, the laser beam is scanned on the surface.
Examples of a thin film of an inorganic material conventionally used as a recording layer of an optical information memory medium are a thin metal film such as an aluminum deposited film, a thin bismuth film, a thin tellurium film, an amorphous film consisting of chalcogenide, and the like. However, since these films have high reflectance and low energy absorbance, a high-output laser is required as a writing light source. In addition, these films have high thermal conductivity and high melting point and hence have low sensitivity. Moreover, elements such as tellurium, bismuth, and serenium are toxious to a human body.
For this reason, instead of thin films of the above inorganic materials, an optical information memory medium using a thin organic pigment film as a recording layer has recently been studied. An optical information memory medium of this type will be briefly described below.
That is, a recording layer consisting of a thin organic pigment film has the following advantages.
An absorption wavelength can be selected and it is possible to obtain a characteristic absorption wavelength within the near infrared region wherein information can be written by a semiconductor laser.
An energy absorbance is high in a write operation.
Thermal conductivity is low.
Productivity is good and toxicity is low.
Typical organic pigments used as the recording layer are exemplified below.
A series of cyanine pigments (Japanese Patent Disclosure (Kokai) No. 58-112790),
A series of anthraquinone pigments (Japanese Patent Disclosure (Kokai) No. 52-224448).
A series of naphthoquinone pigments (Japanese Patent Disclosure (Kokai) No. 58-224793).
A series of phthalocyanine pigments (Japanese Patent Disclosure (Kokai) No. 60-48396).
In order to form a recording layer using the above organic pigments, the pigments are independently used or used as a composition mixed with a auto-oxidizable resin as a binder and coated on the surface of a substrate by application or the like. Examples of a film formation method are a spinner application, dipping, plasma deposition, and vacuum evaporation.
In a spinner application, a pigment is dissolved in a suitable organic solvent to obtain an application solution, and the resultant application solution is dripped on a rotating substrate surface to form a thin film. According to this method, a uniform film can be most easily formed with good productivity. An optical information memory medium using the thin organic pigment film as a recording layer is greatly advantageous because the recording layer can be formed by a spinner application as described above.
Examples of the substrate are a glass substrate and a synthetic resin substrate which are optically transparent with respect to a recording wavelength and do not adversely affect writing/reading. A guide track is formed on the surface of the substrate. The guide track consists of a guide groove (having a width of about 1 .mu.m and a depth of about 0.1 .mu.m) with an optical step and provides a means for tracking write and read positions with high reliability upon scanning.
A guide track can be easily formed by injection molding on a thermoplastic resin substrate. On the contrary, in order to form a guide track on a glass substrate, a difficult molding method such as casting or decalcomania (2P method) using a photocurable resin must be performed, resulting in low productivity. For this reason, injection molded substrates using an acrylic resin, a polycarbonate resin, a polyolefin resin, and the like each of which has good productivity and optical characteristics and is suitable for mass production are most widely used.
However, the optical information memory medium using the organic pigment as the recording layer has the following problems due to an organic solvent contained in an organic pigment solution applied to form the recording layer by spinner application.
That is, as described above, a thermoplastic resin substrate is most widely used as the substrate, and this substrate can be easily dissolved or attacked in an organic solvent except for some alcohol solvents. For this reason, the smoothness of the substrate surface is degraded during a spinner application, or the guide track step is dissolved and vanished. In addition, since the pigment is partially mixed with a substrate resin component, the reflectance of pigment film/substrate interface required for information reading is reduced. As a result, tracking cannot be performed, or the information reading property is degraded.
A glass substrate has high resistance to solvents and hence does not pose the above problems. However, the glass substrate has low productivity, i.e., one of the advantages of the organic pigment memory medium is lost as described above.
In order to solve the above problem, conventionally, an underlying layer with resistance to solvents is provided on the substrate surface, and a pigment solution is applied on the underlying layer to form a recording layer. This underlying layer must have resistance to solvents and must be a very thin film with a thickness of about 10 to 30 nm (0.01 to 0.03 .mu.m) so as not to bury the guide groove with a depth of about 0.1 .mu.m. As such an underlying layer, a film obtained by applying a resin solution of a type which is cured by radiation of light such as ultraviolet rays and curing the solution or an inorganic deposited film such as an SiO.sub.2 film is conventionally used. However, these conventional underlying layers have the following problems.
That is, most of the resins of a type cured by radiation of light are cured by radical polymerization. If the film is very thin, the generation efficiency of radicals by radiation of light is low, and the generated radicals tend to be scavenged by oxygen in the air. For this reason, if a radiating time is not enough, the resin is cured incompletely. On the contrary, if the radiating time is too long, the substrate generates heat to cause thermal deformation.
In addition, since the inorganic deposited film such as the SiO.sub.2 film is formed by deposition or sputtering, a batch method must be used in manufacturing steps, resulting in low productivity. Furthermore, a continuous film with a high density which can completely prevent permeation of solvent molecules cannot be easily obtained.